It is known to jointly use a mechanical diaphragm and a variable ND filter which electrically changes a light transmittance as light quantity adjusting means in an image capturing device such as a video camera or a digital still camera that uses an image capturing element (see Patent Literatures 1 to 3).
In Patent Literature 1, it is disclosed to perform an exposure control in such a manner that, up to a first level of a brightness of a photographic subject, a light quantity is adjusted using a diaphragm while fixing the transmittance of the variable ND filter, when the brightness of the photographic subject is between the first level and a second level, the light quantity is adjusted jointly using the diaphragm and the ND filter, and after the brightness of the photographic subject exceeds the second level, the light quantity is adjusted using the diaphragm while fixing the transmittance of the variable ND filter. With this exposure control, a smooth exposure control with good responsiveness may be achieved or unnatural variation in image quality may be prevented.